-LRB- CNN -RRB- -- I was going through my Twitter feed Monday morning when I came across this : `` Happy greatest day of the year , #Boston ! '' And so it is . Or was , until about 2:50 p.m. , when explosions near the finish line of the Boston Marathon transformed a celebration into a scene of carnage .

What matters now , needless to say , are the victims â $ '' the dead , the injured and their families and friends . But if you are looking for some insight into Boston at this horrible moment , it helps to understand why our marathon matters and where it fits into our civic psyche . Why it was , until Monday , our greatest day of the year .

To begin with , Marathon Monday is a holiday â $ '' Patriots Day , an annual commemoration of the Battles of Lexington and Concord . Not everyone has the day off , but many do , including schoolchildren , for whom this is the first day of spring vacation . That 's why so many people descend on the marathon every year , and why so many people found themselves in harm 's way .

In a city and a region obsessed with tradition , you could n't ask for more than the oldest marathon in the country coinciding with patriotic re-enactments of colonials versus redcoats , all playing out against the backdrop of the budding spring . Today was cool but sunny , and -- finally -- with not a filthy black snowbank in sight . It 's a time of year that gives us hope of better days to come .

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To complete the celebration , Patriots Day marks the annual 11 a.m. Red Sox game at Fenway Park , an utterly local tradition concocted so baseball crowds would n't get in the way of marathon crowds . The runners stream through Kenmore Square , a block away from where other , considerably better-paid athletes are engaged in more sedentary pursuits . The Sox beat the Tampa Bay Rays 3-2 on Monday . There was n't much time to celebrate . The explosions came less than an hour later .

As a spectator sport , the Boston Marathon probably hit its peak in the 1970s , when local favorites Bill Rodgers and Joan Benoit Samuelson were dominating the race . These days , it 's more of a pure spectacle , and I doubt many of those watching could name even one of the East African runners who have won in recent years .

To be honest , I have never attended the marathon in person , let alone run in it . My only direct involvement came in the early 1990s , when I worked as editor of the official program for several years . It is , nevertheless , something that all of us here take pride in .

Moreover , just about everyone has a personal connection . We all know people who were running in it , and perhaps we donated to the charitable cause they had registered with so they could gain entry . As the terrible news began to spread , my Facebook feed was filled with messages from people I know telling me they were all right , or asking if someone else had been heard from .

I am not privy to what sort of security arrangements go into planning the Boston Marathon , but there 's only so much you can do along a 26.2-mile route that passes through seven cities and towns in addition to Boston . As I write this , there is some speculation that bombs may have been placed in trash cans . That may or may not prove to be accurate , but can you imagine securing every trash can , every manhole , every nook and cranny from Hopkinton to Boston ?

Sadly , the marathon will not be the same after today . Security will be tightened , and it will transform what had been a joyful and carefree experience into something else . `` Boston is a tough and resilient town , '' President Obama said in his White House remarks Monday evening . And we are . The marathon will go on . The Patriots Day tradition will continue .

But it may also be a long time , if ever , before anyone again refers to this as our `` greatest day of the year . ''

The opinions expressed in this commentary are solely those of Dan Kennedy .

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Dan Kennedy : Before Monday , the Boston Marathon marked city 's annual `` Greatest Day ''

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It brings the marathon , Patriots Day and an 11 : AM Red Sox game ; kids are off from school

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Kennedy : Marathon is 26.2 miles , in future , must every mile be checked for bombs ?

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Celebration of patriotism , pride , Spring 's return irretrievably marred by bomb horror , he says